Various Calorimetric gas sensors, known as Pellistors, are commonly used in measuring combustible gases in the atmosphere. They operate by allowing flammable gas in the air to combust on the surface of a heated catalyst (typically maintained in the temperature range 350-500° C.) and measuring the excess heat generated in this exothermic reaction. The process is carried out within a flameproof housing so as not to transmit any flame to the general environment being monitored. (See, for example, ‘The Pellistor Catalytic Gas Detector’, E Jones, Ch. 2 in ‘Solid State Gas sensors’ Eds. P T Moseley and B C Tofield, Adam Hilger, 1987 and ‘Calorimetric Chemical Sensors’, P T Walsh & T A Jones, Ch 11 in ‘Sensors Vol. 2’, Eds. W Gopel, T A Jones et al, VCH, 1991).
The combustion reaction is usually promoted using a precious metal catalyst such as palladium, platinum or rhodium, which may be dispersed on a refractory support medium (e.g. alumina or zirconia) to improve its activity and longevity. Raising the temperature of the catalyst to the required working level may be achieved using a variety of heater configurations, but by far the most common is to fabricate a bead of catalyst and support material around a small wire coil which can be heated by the passage of an electric current. Typically, the bead would be roughly spherical with an overall diameter of 400-750 microns (depending on the exact design).
Although it is theoretically possible to separate the functions of heating the catalyst and measuring heat produced by the reaction (for example with a separate thermocouple or a remote IR temperature probe), it is most practical to combine these requirements in a single element. Thus, the wire coil is required to act as a resistive heater and demonstrate a significant, reproducible temperature coefficient of resistance so that the temperature increase generated by the reaction can be easily measured. The wire used is most commonly platinum, and may have a diameter of 20-50 microns. A schematic diagram of a known gas-sensitive detector, or sensor element is shown in FIG. 1.
Since the detector is essentially a thermal sensor, designed to respond to changes in temperature, it can be sensitive to numerous effects which are not caused by the combustion of flammable gas, for example:
Changes in ambient temperature;
Fluctuations in the thermal properties of the atmosphere, for example due to relative humidity or other interfering gases;
Changes in air speed which alter the cooling of the hot element;
Variations in the input power to the coil heater due to power supply instabilities.
During the early development of pellistors, it was recognized that compensation for such effects was required in order to provide a practically reliable device and so it became common to deploy the detector in conjunction with a compensator element. In its simplest form, this is an identical device but without the catalyst. In some instances the bead is actually poisoned to minimize gas response. By running the detector and compensator elements in a Wheatstone bridge circuit with balancing resistors Rb as illustrated in FIG. 2, it is possible to generate an out of balance signal which substantially depends solely on the concentration of flammable species in the environment.
Such pellistor pairs have been a technical and commercial success, delivering major improvements in health and safety in many industries (e.g. mining, petrochemical processing). Improvements in bead construction, catalyst chemistry and housing design have resulted in stable, sensitive, poison-resistant products capable of operating for many years with minimal maintenance. In recent years it has become common to package the pellistor pair inside a small robust flameproof housing which can be readily incorporated into a variety of instruments or systems (e.g. 4P75, MICROpeL®75, City Technology Ltd, Portsmouth, UK).
Despite this successful background, there are still some aspects of pellistor performance where users seek improvements. Foremost of these is the power consumption—even the smallest commercial bead pairs consume in excess of 200 mW. While this is not a problem for fixed installations, in portable multi-gas responsive detectors, for example detectors which measure various different gases such as oxygen, carbon monoxide, and hydrogen sulfide using electrochemical sensors or cells, in addition to measuring an explosive gas, such as methane using a pellistor pair, the 200 mW of power consumed by the pellistor pair is a large fraction of the total instrument consumption and so limits the operational period between battery recharges.
A number of approaches to solving this problem have been tried, including:
Intermittent or cyclic operation of the detector/compensator pair so that they only run at full power for a fraction of the operating period;
Single element detector operation at different temperatures to separate the effects of interferences and flammable gases;
Single element detector operation without compensation.
The above noted methods fail to meet the stringent demands placed upon pellistors. It has been found that any method in which the detector is regularly subjected to temperature cycling tends to promote drift in the output, necessitating more frequent recalibration. In the vast majority of safety-critical applications, an uncompensated detector output is wholly incapable of meeting accuracy specifications, given the wide temperature (−40 to +55° C.) and humidity (0-100% RH) ranges over which the devices are required to operate.
However, a number of the factors which the compensator was originally designed to address have been overcome in other ways. Thus, the incorporation of elements into flameproof housings (often surrounded by porous insulators to improve shock resistance), has greatly reduced the flow rate sensitivity. Modern electronic circuits are able to provide highly reliable and accurate power supplies at relatively low cost. Therefore, the major functions of the compensator in modern designs are the correction for ambient temperature and RH as well as correction for changes in the thermal conductivity of the background atmosphere. However, this is achieved at a relatively high power consumption and significant cost (once the bead construction and pair matching are taken fully into account).
Temperature and RH changes in the environment which require compensation generally occur over a timescale which is relatively long compared with the alarm period of a pellistor (T90 typically 30 seconds). Thus, continuous operation of the detector with intermittent compensator powering to provide periodic correction may be employed as in the MICROpeL®40 sensor (City Technology Ltd, Portsmouth, UK). This device offers up to 40% power savings when compared with conventionally operated pellistors in the same housing. However, intermittent compensator operation can be complicated to implement and requires careful (and more complex) hardware and software design to ensure correct interpretation of data. Despite the advantages offered, this approach is not acceptable to all users and so an alternative is required.
There continues to be a need for lower power pellistor based detectors of the general type described above. Preferably, lower power operation will be achievable via methodologies which do not require the types of complexities discussed above.